@marty I think it is based on how auras are catagorized. For the paladin, anything that affects enemies is clustered under offensive, as defensive auras help out party members. But in the case of a merc, holy freeze is very much defensive as it slows down enemies, helping out the player that they are protecting.
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ResorathMay 25 '11 at 6:29

@Oak has this completely in the bag. One more thing to keep in mind that is that mercs are often used as an additional set of gear slots for the purpose of auras - there are some incredible higher-level runewords that grant some incredible auras. So keep in mind that you could get a fairly high-level Might or Fanaticism aura from a particular runeword when selecting which merc you want - it can be quite a pain to get a merc up to higher levels.